. Stereotomy : Problems in stone cutting. In four classes. I. Plane-sided structures. II. Structures containing developable surfaces. III. Structrues containing warped surfaces. IV. Structures containing double-curved surfaces. For students of engineering and architecture . 2°. Make drawings of a key-stone, extending both ways from a in each arch. 3°. Also of an inner pier groin stone, as Mi/niQi. 4°. Construct the plane joints indicated in 12°. 5°. Make the necessary illustrative drawings of the double-curved arched cover-ing of circular stairs, known as the St. Giles screw (137). The coursin
. Stereotomy : Problems in stone cutting. In four classes. I. Plane-sided structures. II. Structures containing developable surfaces. III. Structrues containing warped surfaces. IV. Structures containing double-curved surfaces. For students of engineering and architecture . 2°. Make drawings of a key-stone, extending both ways from a in each arch. 3°. Also of an inner pier groin stone, as Mi/niQi. 4°. Construct the plane joints indicated in 12°. 5°. Make the necessary illustrative drawings of the double-curved arched cover-ing of circular stairs, known as the St. Giles screw (137). The coursing joints areoblique helicoids, generated by radii of the semicircle ac^b, Fig. 73. The headingjoints are in planes perpendicular to the helix generated by the highest point, &i,of the same semicircle. The coursing joints thus generated are not quite normalto the double-curved arch surface, but are nearly enough so. To be perfectly so,they should be generated by normals to that intrados, that is, by lines, not onlyperpendicular to the tangents to the semicircle, as at its points of division, etc.,but also to the tangents to the helices at the same points. The construction wouldbe too laborious, and thence more apt to be inexact. PI. IX M|- v t: t if ! V B Y. , 6a i i v, ■ 10 Q I GA ]
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